Workers in the art of making integrated circuit devices are well aware of the need for optimal techniques for developing silicon oxide films on an integrated circuit (wafer) substrate. This invention is intended to provide improved techniques for growing a silicon oxide film on such substrates.
Silicon dioxide has played a major role in the fabrication of silicon [micro-electronic] devices and their operation since approximately 1958. If a wafer of silicon is heated in an atmosphere of oxygen or water vapor, a film of silicon dioxide is readily formed on its surface. This film is hard, durable [e.g., softening about 1400.degree. C.] and it firmly adheres to the silicon substrate. It makes an excellent electric insulator and is very convenient to use in the fabrication of integrated circuits, serving as a mask for the selective introduction of dopants.
Convenient thicknesses of silicon dioxide can easily be grown in an oxidizing atmosphere at temperatures on the order of 1000.degree. to 1200.degree. C.--thickness being rather precisely controlled by selecting the appropriate time and temperature of oxidation. For example, a 0.1 micrometer layer of oxide will grow on a Si wafer exposed to an atmosphere of pure oxygen for about one hour at a temperature of 1050.degree. C. [substituting steam, for the pure oxygen, will grow a layer five times as thick]. As workers well known, such pyrogenic oxidation is popular for growing such precision silicon dioxide films; it is very convenient and inexpensive to use (several hundred wafers can be simultaneously oxidized in a single run).
More particularly, workers are aware that during the preparation of certain types of integrated circuits, the silicon wafer substrates are oxidized to establish an SiO.sub.2 film adjacent silicon nitride mask areas (e.g., to place a thick "Field Oxide" there). It has been found that, in the course of growing such an oxide film (pyrogenically), a deleterious "white ribbon" effect can commonly occur injecting an undesired silicon nitride film under their mask areas. Such "white ribbon" films are commonly formed at a silicon/silicon oxide interface, where an SiO.sub.2 layer (thermal buffer) underlies the nitride mask during pyrogenic oxidation accompanied by the presence of nitrogenous products (e.g., believed to include ammonium evolved from the nitride mask; see article by Kooi, et al. page 1117 et seq., Volume 123 of Journal Electro-Chemical Society, 1976).
Such "white ribbon" films have become familiar as potentially disadterous to device yield. As explained below, they are believed to represent the reaction product of ammonium with silicon and constitute a thin silicon nitride film that is quite difficult, expensive and inconvenient to remove [e.g., a special etching might be carried out, but is contra-indicated because of the added time and expense it would involve and because it would likely damage the SiO.sub.2 film and the Si substrate.]
It is believed, though not certain, that the nitrogen constituent of the "white ribbon" film finds its way to the film-site by diffusing through the silicon oxide to reach the silicon substrate. Nitrogen is readily diffusible through SiO.sub.2 at the prevailing high ambient temperatures, and ammonia gas would appear to be more effective in producing this nitride than molecular nitrogen. However, it is not certain whether these and/or some other "N entity" are operative.
The resultant nitride film has a "masking effect" against oxidation and thus will interfere with the superposition of an oxide layer. For instance, a "white ribbon" spot will obstruct a "gate oxide" of a MOS structure, making it too thin and resulting in unacceptably low gatebreakdown-voltage and device rejection.